The deposition of thin film on the surface of a semiconductor wafer is a required step in the manufacture of a semiconductor device. Since the thickness of the thin film greatly effects the characteristics of the semiconductor device, this thickness must be precisely controlled.
The conventional method for regulating the thickness of a film is typically performed following the completion of the deposition of the film by CVD. A dedicated wafer for monitoring the thickness is taken from the apparatus and its film thickness is measured by a second measurement tool. The deposition rate is then calculated as a function of the film thickness and the time spent on the deposition. Based on this calculated rate, the deposition time of the next batch (i.e., the next film) is adjusted to form a film on the surface of the wafer having the calculated thickness. This is referred to as a post-performance method. In order to alter the thickness of the film, a monitor wafer in each batch is employed when using LPCVD. Upon completion of the formation of the film in batch processing, the thickness of the film on the monitor wafer is examined to compare its thickness to the predetermined value. The difference in thicknesses obtained from a preceding process is feedback to adjust the deposition rate for a succeeding batch. Since this approach is a post-performance film thickness adjustment method, changes in film thickness on product wafers will be significant as the film thickness varies greatly from batch to batch. Clearly, this method is inaccurate and time consuming.
In a second film formation control method, a change or increase in thickness of a film is monitored in-situ during batch film deposition. To monitor the thickness of the deposited film using CVD, an externally introduced laser beam is projected through a quartz furnace to the surface of a monitor wafer. Variations in strength of the reflected light is used to determine changes in thickness of the film. In Japanese Unexamined Patent Publication No. Sho 62-173711, a CVD step is described during an optical process, wherein a thin film is laid on the surface of the substrate. With this method, the thickness of the film is monitored using the laser beam. The laser beam originating outside the reaction furnace is channelled through a light transparent port and is projected onto the surface of material inside the furnace. The laser beam reflected by the substrate passes through the port and is collected outside the furnace. In the cited reference, however, no consideration is given to the problem related to the effect produced by the film that is attached to the port due to the presence of the incident and reflected laser beams traversing the port. This method is referred to as epochal film thickness control method.
Standard CVD apparatus using a heater surrounding the quartz furnace defines a region in which gas reacts such that inside the region, a portion is formed which satisfies the condition for film deposition of providing the necessary temperature for decomposition of the reactive gas. At this time, the internal wall of the quartz furnace satisfies the conditions for film deposition and a film is deposited thereon. This deposited film absorbs, reflects, or interrupts the laser beam. As a result, it is difficult for a change in thickness of a film on the surface of the wafer to be precisely detected by changing the intensity of the reflected laser beam. To resolve this shortcoming, each time the film formation process is completed, a cleaning gas is introduced into the quartz furnace to remove film deposited inside the furnace. In this case, as the operating rate for the CVD apparatus is reduced, it results in an increased manufacturing cost. Thus, this method is not deemed to be practical. Furthermore, when a wafer is irradiated with a laser beam while the film is being deposited inside the quartz furnace, the laser beam must pass through a region wherein no film is deposited, a clear added drawback.